Our '87 300E (automatic) gets 15-18 mpg city and 20-22 mpg highway.
The importance of a thorough inspection of any used car cannot be overemphasized, but especially with Benzes. Component build quality, even for Mercedes, varies across and within model years, never mind the variables of service and maintenance after the initial purchase. A "good" benz will have very low maintenance costs, a "bad" benz will have very high maintenance costs.
Depending on the policies of the shop you use, the cost of troubleshooting obscure problems can also be fairly expensive. Find a good shop, treat them well, and/or develop a good diagnostic sense of your own.
As a personal example, we bought our '87 300E new, and while it's held up fairly well over the years, it has needed a number of expensive jobs outside of the normal maintenance routine. The ones that come most immediately to mind were the requisite valve work (around '95 I believe) and a rebuilt transmission (last month). It is also due to have all ten rear suspension links replaced, along with the rear diff mount and the tie rods (according to an initial analysis by our dealer), which I imagine will not be cheap.
The car has also stranded a driver on three separate occasions (luckily, two were in our garage) - first thanks to an alternator, then a worn ignition key tumbler mechanism, and finally (last week actually) a dead distributor/rotor assembly. Early on, we had a number of electrical problems (window lifters and seat motors, mostly), and the AC quit for several years starting around '94. It has also consumed/leaked oil (in varying amounts) for a long long time. I think we're back to a quart every 1200 miles or so (from an all-time high of a quart every 4-600 miles).
OTOH, given its age, the car has held up pretty well. It's survived a couple of extended road trips, a flooding, a very high percentage of city miles, a fair number of minor car-to-object scuffles, and lots of delayed (or forgotten) repairs. It's still a joy to drive (well, besides the squirrelly rear - see suspension work above), and I think we'll keep it for quite some time to come.
As always, your mileage will vary.